1. Field of the Invention
This invention relates to retrofocus type lenses and, more particularly, to a retrofocus type lens suited to a projection lens for a color liquid-crystal projection television system, in which, for example, a plurality of images having different pieces of color information are combined by using composition mirrors and then projected onto a screen in an enlarged scale.
2. Description of the Related Art
Various color liquid-crystal projection television systems of such a kind that images displayed on a plurality of color liquid-crystal display elements (liquid-crystal light bulbs) are optically superimposed and projected by a projection lens onto the screen, have been proposed.
FIG. 7 is a schematic view of the main parts of a general type of the color liquid-crystal projection television system in which the images formed in the color liquid-crystal display elements are projected onto the screen (not shown).
In FIG. 7, a white color light source 1 issues a bundle of collimated light rays. Liquid-crystal display elements 2a, 2b and 2c for red, green and blue respectively present images to be projected. Reference numerals 3a and 3b denote reflection mirrors. A red-reflection dichroic mirror 4 illuminates the liquid-crystal display element 2a for red. A green-reflection dichroic mirror 5 illuminates the liquid-crystal display element 2b for green.
The liquid-crystal display element 2c for blue is illuminated by the blue light which has passed through the red-reflection dichroic mirror 4 and the green-reflection dichroic mirror 5. Reference numeral 6 denotes a blue-reflection dichroic mirror. Reference numeral 7 denotes a projection lens.
In FIG. 7, white light issued from the light source 1 is separated into red light, green light and blue light by the dichroic mirrors 4 and 5. With the red light, green light and blue light, the liquid-crystal display elements 2a, 2b and 2c for red, green and blue are illuminated, respectively. Images formed on the liquid-crystal display elements 2a, 2b and 2c with these color rays of light are projected by the projection lens 7 in a superimposed relation on the screen (not shown) to obtain a color picture.
For the projection lens 7 employed in the above arrangement, a retrofocus type lens having a long back focal distance is frequently used in view of the necessity of disposing various kinds of optical members such as reflection mirrors and dichroic mirrors in a space between the last lens surface and the liquid-crystal display elements (corresponding to the back focal distance).
In general, the retrofocus type lens is constructed in the form comprising a lens unit of negative refractive power on the object side (the side of a conjugate point of long distance) and a lens unit of positive refractive power on the image side (the side of a conjugate point of short distance). For this reason, there is an advantage that a relatively long back focal distance is easily obtainable.
Since the lens configuration is asymmetric, however, there is a tendency to produce many asymmetric aberrations such as distortion and astigmatism.
Further, in a case where the retrofocus type lens is employed in the color liquid-crystal projection television system, because the dichroic mirrors used for color separation are disposed on the image side, in order that the color unevenness is removed to effect good reproduction of colors over the entire area of the picture frame, there is need to make an almost equal angle of incidence of the light beam on the dichroic mirror over the entire area of the picture frame. Thus, the lens system must be made almost like the so-called emergence telecentric system in which a principal light ray emerging from the image side is almost parallel to the optical axis.
However, the use of the emergence telecentric system results in the off-axial light beam entering the lens unit of positive refractive power on the image side at a high position above the optical axis. For this reason, a problem arises in that large barrel-type (negative) distortion and flare are produced so that it is very difficult to obtain a good projected image.
In Japanese Laid-Open Patent Application No. Hei 3-145613, there has been proposed a retrofocus type lens having a lens form similar to the emergence telecentric system suited to a projection lens used for the color liquid-crystal projection television system in which the above-described distortion and flare and other various aberrations are corrected by setting forth proper rules of lens design.